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United States Patent |
5,047,784
|
Gerlach
,   et al.
|
September 10, 1991
|
Zero cross-correlation complementary radar waveform signal processor for
ambiguous range radars
Abstract
A method and apparatus exploiting the discovery that the crosscorrelation
of constantly spaced rows of the matrices representing certain pulse codes
sum to zero. In a ranging system, such as a radar, pulses are coded
according to the rows of a such a matrix, transmitted sequentially and
each return processed sequentially through a filter matched to one of the
coded pulses. (A different preselected filter is used for each return.)
The sequence of filters is chosen so that for returns for a given range
interval, each filter is matched to the returning pulse, resulting in
outputs from the filters representing auto-correlations of the returned
pulses. These outputs are time delayed added coherently to form the
compressed pulse, and annunciated as a target hit. Should the filters and
returns be mismatched, as with ambiguous stationary clutter returns, the
outputs of the filters are cross-correlations which, according to said
discovery, sum to zero. Thus the invention operates to remove ambiguous
range clutter from returns in such a ranging system.
Inventors:
|
Gerlach; Karl R. (Dunkirk, MD);
Kretschmer, Jr.; Frank F. (Sarasota, FL)
|
Assignee:
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The United States of America as represented by the Secretary of the Navy (Washington, DC)
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Appl. No.:
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647946 |
Filed:
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January 30, 1991 |
Current U.S. Class: |
342/201; 342/132; 342/134; 342/145; 342/204 |
Intern'l Class: |
G01S 013/30 |
Field of Search: |
342/145,134,201,132,202,204
|
References Cited
U.S. Patent Documents
H767 | Apr., 1990 | Kretschmer, Jr. et al. | 342/145.
|
Other References
Karl Gerlack and F. F. Kretschmer, Jr., General Forms and Properties of
Z Cross-Correlation Radar Waveforms, Naval Research Laboratory,
Washington, D.C., NRL Report 9120, Jan. 30, 1990.
|
Primary Examiner: Sotomayor; John B.
Attorney, Agent or Firm: McDonnell; Thomas E., Miles; Edward F.
Claims
We claim:
1. A method of ranging comprising steps for:
transmitting a sequence of coded pulses F.sub.1, F.sub.2, . . . ,
F.sub.(N-1), F.sub.0, F.sub.1, . . . , F.sub.N-1, each of said coded
pulses F.sub.n, n=0, 1, 2 . . . , N-1, being coded in accordance with the
(n+1)th row of a matrix of dimension NxN given as follows:
##EQU4##
each of said coded pulses being spaced from adjacent ones of said coded
pulses by time intervals t.sub.0, t.sub.1, . . . , t.sub.N-1, each F.sub.n
being followed immediately by a corresponding t.sub.n, the last N of said
time intervals being denominated detection intervals;
selecting an integer c from the set whose members are: 0, 1, 2, . . . ,
N-1;
detecting returns of said coded pulses during each of said detection
intervals;
passing said returns detected in each said detection interval through a
corresponding filter matched to one of said coded pulses F.sub.n,
n=(N+j-c) mod N, j=0, 1, 2, . . . , N-1;
coherently summing the outputs of said filters generated during said
detection intervals.
2. An apparatus for ranging, said apparatus comprising:
means for transmitting a sequence of coded pulses F.sub.1, F.sub.2, . . . ,
F.sub.(N-1), F.sub.0, . . . , F.sub.n-1, each of said coded pulses
F.sub.n, n=0, 1, 2, . . . , N-1, being coded in accordance with the
(n+1)th row of a matrix of dimension NxN given as follows:
##EQU5##
said means for transmitting being effective to cause each of said coded
pulses to be spaced from adjacent ones of said coded pulses by time
intervals t.sub.0, t.sub.1, . . . , t.sub.n-1, each F.sub.n being followed
immediately by a corresponding t.sub.n, the last N of said time intervals
being denominated detection intervals;
means for selecting an integer c from the set whose members are: 0, 1, 2, .
. . , N-1;
means for detecting returns of said coded pulses during each of said
detection intervals;
means for passing said returns detected in each said detection interval
through a corresponding filter matched to one of said coded pulses
F.sub.n, n=(N+j-c) mod N, j=0, 1, 2, . . . , N-1;
means for coherently summing the outputs of said filters generated during
said detection intervals.
Description
BACKGROUND OF THE INVENTION
In simplest form, a radar system consists of the generation of a pulse
having a certain duration, followed by a listening period in which returns
are received. A radar designer usually wishes to increase the power of
target returns to provide better detection. The most straightforward way
to do this is to increase pulse amplitude. Unfortunately, useful radars
require pulse amplitudes that would result in waveguide arcing and
electrical breakdown. A conventional way to circumvent this problem is to
use pulse compression techniques, i.e. transmitting a series of low
amplitude pulses (subpulses) of the same aggregate energy as a higher
amplitude pulse. The pulses are typically modulated (the modulated pulse
also called a coded waveform) and transmitted. Returns are processed
through a matched filter (i.e. a filter whose transfer function optimizes
the signal to noise ratio), resulting in a signal that is a compressed
pulse that is also the auto-correlation of the coded waveform in the
absence of doppler shifts. Pulse coding can be expressed in matrix form,
examples of which are matrices for the well-known Frank and P4 codes. The
matrix describes the phase shifting necessary to phase modulate the
subpulses of a coded pulse. Such a matrix is a square one of dimension
NxN, each element of which represents a phase shift (the phase modulation)
of one subpulse. The Frank or P4 code consists of concatenated N.sup.2
subpulses having the phases described by the elements of the consecutive
rows of the matrix, reading from left to right. An example of a Frank
matrix is shown in FIG. 1 for N=4. The elements of, e.g., the second row,
1, j, -1, -j, describe the fifth to eighth subpulses with respective phase
modulation of 1 (i.e. 0.degree.), j (i.e. 90.degree.), -1 (i.e.
180.degree.), and -j (i.e. -90.degree.).
Such a radar system commonly operates by generating a sequence of
identically coded waveforms, separated in time by detection, or listening
intervals, in which the radar can detect returns of the transmitted
waveform. The range for which the radar can receive unambiguously is
limited to the distance a pulse can travel to and from the radar during
its detection interval. This distance is called the unambiguous range.
Often, downrange from the unambiguous range is clutter (e.g. hills) which
can reflect radar returns, and such clutter can cause pulses to return to
the radar during detection intervals for later pulses (i.e. be "folded
over" into a later detection interval). Clutter causing foldover into the
next pulse's detection interval is said to be located in the first
ambiguous range, foldover into the second succeeding detection interval is
said to be from the second ambiguous range, etc. Unambiguous range clutter
is undesirable because it increases the cancellation requirements of the
radar and the dwell time required to process clutter returns, and because
it causes the range to be ambiguous in mapping applications.
In Statutory Invention Registration (SIR) H767, the inventors disclosed a
method and apparatus for eliminating ambiguous range clutter. The
invention of SIR H767 derives from a discovery by the inventors of
properties of the Frank and P4 matrices, in particular that the sum of
cross-correlations between rows of a Frank or P4 matrix, spaced by a
constant number of rows, is zero. More generally, for such a matrix of
dimension NxN, if the cross-correlations between rows q and m of the
matrix are given by C.sub.qm (i), for i=.+-.0, .+-.1, .+-.2, . . . ,
.+-.(N-1):
##EQU1##
where m=(q+r) mod N, r=1, 2, . . . , (N-1).
The invention of SIR H767 is a method and apparatus for transmitting and
processing a sequence of coded pulses F.sub.1, F.sub.2, . . . , F.sub.N-1,
F.sub.0, F.sub.1, . . . , F.sub.N-1, each of the coded pulses F.sub.n,
n=0, 1, 2 . . . , N-1, being coded in accordance with the (n+1)th row of a
Frank or P4 matrix of dimension NxN. Each of the coded pulses are spaced
from adjacent ones of the coded pulses by time intervals t.sub.0, t.sub.1,
. . . , t.sub.n-1, each F.sub.n being followed immediately by a
corresponding t.sub.n, the last N of said time intervals being denominated
detection intervals. An integer c is selected from the set whose members
are: 0, 1, 2, . . . , N-1. Returns of the coded pulses during each of the
detection intervals are detected. The returns detected in each detection
interval are passed through a corresponding filter matched to one of the
coded pulses F.sub.n, n=(N+j-c) mod N, where j=0, 1, 2, . . . , N-1. The
outputs of the filters generated during all N detection intervals are
coherently summed.
The importance of this scheme derives from the inventors' discovery that
the cross-correlations of rows of Frank or P4 matrices spaced equally
apart sum to zero. In most simple form, such a system is designed to
generate a series of pulses F.sub.0, F.sub.1, F.sub.2, . . . , F.sub.n-1.
After each pulse the system processes returns using a filter matched to
the pulse, changing the filter with each detection interval. Thus over all
the detection intervals the system employs a sequence of filters matched
to the various pulses F.sub.n, and employs them in the same order as the
pulses to which they are matched. Each value of c shifts the filter
sequence in a circular manner, and clutter time shifts the returns an
amount determined by the particular ambiguous range in which the clutter
is situated. If these shifts are identical, each returning pulse in each
interval is matched to the filter employed, and the detected signal in
each interval is the auto-correlation of the filter's transfer function.
The coherent sum of these auto-correlations over the detection intervals
yields the compressed pulse. If the shifts are not identical, the detected
output in each interval is the cross-correlation of the pulse and the
transfer function of the filter. Because the pulses are coded sequentially
according to rows of a Frank or P4 matrix, and because the filters are not
matched to these pulses in this sequence, the coherent sum of these over
the detection intervals constitute the sum of cross-correlations between
rows of the coding matrix spaced a constant amount apart. The inventors'
discovery about Frank or P4 matrices demonstrates that this sum is zero.
Thus by choice of c a system according to the invention can "tune" itself
to detect returns from the unambiguous range, or any of the ambiguous
ranges, and reject all other returns. One could also have a plurality of
these systems, each tuned to one range, and thus detect all returns and
simultaneously determine from which range each return has come.
However, the invention disclosed in SIR H767 is limited to use with a Frank
or P4 code. This limits the freedom an engineer has in designing radar
systems which have the advantages of that invention. Also, with the Frank
or P4 code even small doppler shifts from ambiguous range echoes can be
detected, causing false alarms.
SUMMARY OF THE INVENTION
Accordingly, an object of the present invention is to be able to reduce or
eliminate ambiguous range clutter in radar systems which use a much
broader range of pulse codes than just the Frank or P4 codes, so as to
give the design engineer greater flexibility in designing such radars.
Because the ability to distinguish clutter from non-clutter implies also
the ability to map the clutter, another object is to identify the
existence of non-ambiguous range clutter so as to facilitate existing
techniques to map clutter.
In accordance with these and other objects made apparent hereinafter, the
invention is a method and apparatus for ranging like that of SIR H767,
except that the pulses are coded in accordance with the (n+1)th row of the
NxN matrix F, having rows F.sub.0, F.sub.1, . . . , F.sub.(N-1), where the
elements of F are given by:
##EQU2##
and where: n=0, 1, 2, . . . , (N-1),
d.sub.n is an arbitrarily chosen complex number of unit magnitude for all
n,
.lambda. is chosen from the set whose members are {W.sub.N.sup.n },
W.sub.N =exp(j2.pi./N) , and
M is an integer relatively prime to N.
The inventors have discovered that, like the Frank or P4 codes, the sum of
the cross-correlation functions between equally spaced rows of the matrix
F is zero, i.e.:
##EQU3##
Where C indicates the correlation function. This is the same property which
permitted the clutter reduction in SIR H767. However, F above comprehends
a much broader class of codes than merely the Frank and P4 codes. (For
example, for d.sub.n =1 for all n, .lambda.=1, and M=1, F reduces to the
Frank code.) Because one has a relatively wide range of choices in
selecting the values for .lambda. and the d.sub.n 's, the radar engineer
has greater flexibility in systems design. Variations in the phase codings
imposed on coded pulses can vary such system characteristics as sidelobe
magnitude, and sensitivity to doppler shifting of echoes. Because .lambda.
and the d.sub.n 's are N+1 system variables which can vary independently
of one another, the invention gives the engineer designing a radar system
N+1 degrees of freedom in selecting a set of codings which will also
secure for the system the advantages disclosed in SIR H767. The engineer
can use the N+1 degrees of freedom to select (e.g. iteratively) a set of
codings for any particular application which will optimize desired system
characteristics, such as sidelobe magnitude or sensitivity to doppler.
(See, K. Gerlach et al., "General Forms and Properties of Zero
Cross-Correlation Radar Waveforms," NRL Report 9120 (Jan. 30, 1990), the
text of which is incorporated herein by reference.
The invention is more fully understood from the following detailed
description of a preferred embodiment, it being understood, however, that
the invention is capable of extended application beyond the precise
details of the preferred embodiment. Changes and modifications can be made
that do not affect the spirit of the invention, nor exceed its scope, as
expressed in the appended claims. Accordingly, the invention is described
with particular reference to the accompanying drawings, wherein:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1a is an example of a Frank matrix of dimension 4x4.
FIG. 1b is the code used in the invention, presented in the form of matrix
F.
FIG. 2 is a schematic representation of the coded pulses used in a four
channel embodiment of the invention.
FIG. 3 is a schematic diagram of a processing system used with the
invention.
FIG. 4 is a schematic diagram of an alternative processing system according
to the invention.
FIG. 5 is a table listing the sequence of matched filters used in the
invention.
DETAILED DESCRIPTION
With reference to the drawing figures, like numbers indicating like parts
throughout the several views, and with particular reference to FIG. 2,
this figure shows schematically a radar waveforms using four coded pulses
F.sub.0, F.sub.1, F.sub.2, F.sub.3. These coded pulses could be those
corresponding respectively to the rows of the matrix of FIG. 1b for N=4
(i.e., a 4.times.4 matrix). (The choice of N=4 is arbitrary, and is done
here for clarity of explanation.) After each coded pulse F.sub.n is
transmitted, the system "listens" for a period t.sub.n for returns of
F.sub.n. If target range is chosen properly, and no ambiguous range
clutter is present, a return of F.sub.0 should occur in t.sub.0, a return
of F.sub.1 in t.sub.1, etc. These returns are time delayed and coherently
added by conventional circuitry. (Cf. FIGS. 3 and 4.)
The presence of ambiguous range clutter can cause "folding over" of returns
of one coded pulse F.sub.n into the wrong detection interval t.sub.m, m=n,
resulting in spurious range returns. For example, a clutter return from
the second ambiguous range would cause a coded pulse F.sub.0 to arrive
within t.sub.2, F.sub.1 to arrive in t.sub.3, etc.
This is countered by first adding additional coded pulses F.sub.1, F.sub.2,
F.sub.3 (or, more generally, F.sub.1, F.sub.2, . . . , F.sub.N-1). These
additional pulses are indicated, respectively as members 10, 20, 30 in
FIG. 2. Additional pulses 10, 20, 30 ensure that, if there is folding
over, each detection interval t.sub.n will have a coded pulse folded into
it. For example, even with folding over, over all four detection intervals
(more generally N detection intervals) a complete set of all four (more
generally N) coded pulses will return. This is necessary for application
of the inventors' discovery about the class of codes shown in FIG. 1b.
FIG. 3 shows schematically a system according to the invention. Antenna 40
receives a return which is passed by interlock 44 to a filter 46.
Interlock 44 has conventional electronics (not shown) to steer the return
to a preselected one of filters 46 during each detection interval. The
filtered return passes to a second interlock 48 which contains
conventional circuitry (not shown) to select an appropriate time delay 52
to permit coherent summing of the return by summer 56 over a complete set
of detection intervals. In operation, interlock 44 sequences the choice of
filters 46 to "tune" the system to a particular ambiguous (or the
unambiguous) range as above described.
FIG. 4 shows an alternative arrangement having a plurality of legs 42, each
constituted by a system as shown in FIG. 3. In particular, each leg 42 has
a bank of matched filters 42 (corresponding to members 44 and 46 of FIG.
3), and delay and sum processors 60 (corresponding to members 48, 53, and
56 of FIG. 3). In this arrangement, the sequence of individual filters 46
in banks 50 is done to "tune" each leg 42 to a particular one of the first
three ambiguous ranges, and the unambiguous range, respectively. In this
way, all returns from these ranges can be detected simultaneously, and,
depending on which leg detects the return, can identify the range of
origin.
The invention has been described in what is considered to be the most
practical and preferred embodiments. It is recognized, however, that
obvious modifications may occur to those with skill in this art.
Accordingly, the scope of the invention is to be discerned solely by
reference to the appended claims, wherein:
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